This invention relates to low supply voltage electronic circuits, particularly low power integrated circuits where a voltage higher than, or of opposite polarity to the supply voltage is required for proper operation.
Charge pumping circuits are used to generate a voltage with a magnitude larger than the supply voltage. FIG. 1 shows such a circuit commonly known as a voltage doubler. A signal generated from the OSCILLATOR appearing at point 1 is inverted by the inverter INV1. The inverted signal appearing at the output 2 of INV1 is a saturated signal with a magnitude approximately equal to the supply voltage Vcc of the oscillator. The inverted ac signal at point 2 charges CA to the peak value of the ac signal through the MOS diode MN2 during one half-cycle. During the other half-cycle CA discharges in series with the ac signal at point 2 through MOS diode MN1. Thus, the voltage at point 3 and VBB at point 4 are ideally equal to -Vcc.
This negative VBB can be added to the positive Vcc to obtain a voltage equal to the absolute sum of VBB and Vcc. However, due to the finite threshold voltage Vth of the MOS diodes, the voltage at point 3 is equal to (-Vcc+Vth) and the voltage VBB at point 4 is equal (-Vcc+2Vth). When the supply voltage Vcc is low, this negative voltage may not be sufficient.